>>7293867Because if you destroy Reimu, you destroy the Great Hakurei Barrier. And if you destroy the Great Hakurei Barrier, then you bring calamity upon Gensokyo.
And if you bring calamity upon Gensokyo, you bring the wrath of Yukari upon yourself.
I don't care how good Sakuya is at manipulating time and space. Yukari can manipulate the fucking BORDERS of time and space. She can be anywhere and everywhere at anytime or every time. In short, Sakuya would be fucked.
Besides, I'd assume that using her powers so flippantly would drain Sakuya greatly. That, and she bears no grudge against Reimu.
... And if you give a mouse a cookie, he's going to want a glass of milk to go with it.